Essential package 'ubuntu-minimal' could not be located

[PabloAB]

While upgrading from bionic to focal fossa I get

> Invalid package information
>
> After updating your package information, the essential package 'ubuntu-minimal' could not be located. This may be because you have no official mirrors listed in your software sources, or because of excessive load on the mirror you are using. See /etc/apt/sources.list for the current list of configured software sources. In the case of an overloaded mirror, you may want to try the upgrade again later.

Just like [this user from Askubuntu](https://askubuntu.com/q/1231817/349837)

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-118.119-generic 4.15.18
Uname: Linux 4.15.0-118-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
Date: Tue Sep 29 17:38:17 2020
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-09-29 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2020-09-29 17:30:01.275265
 Log time: 2020-09-29 17:30:13.209067
 Log time: 2020-09-29 17:37:16.332964
 Log time: 2020-09-29 17:38:21.899194
VarLogDistupgradeTermlog:

[PabloAB (pabloab777)]

After making the changes suggested on the question (for bionic) the error is now

> Could not calculate the upgrade

Tried [this](https://askubuntu.com/a/360445/349837), but python-minimal have too many dependencies.

    2020-09-29 18:07:04,656 INFO failed to remove python-minimal
    2020-09-29 18:07:05,349 ERROR Dist-upgrade failed: 'E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.'